Browsing and grazing ruminants: Are they different beasts?

Abstract

Ruminant species are classified into three main feeding categories: those which feed mainly on browse material (browsers); those which feed mainly on grass (grazers); and those which feed on a mixture of the two plant types (mixed or intermediate feeders). Much literature has accumulated on the morphological and physiological adaptations, which have evolved in the different ruminant feeding categories to efficiently extract the nutrients from the diet consumed. These include adaptations of the mouth and the digestive system. Recently, there has been a number of re-analyses of the data which show that there is little substantive evidence for differences in morphology and physiology between the feeding categories, once differences in body mass and phylogenetic relationships have been taken into account. However, the hypotheses linking food, form and function should not be dismissed out-of-hand until better quality experimental hypothesis testing has been conducted. In the past, the feeding behaviour required to glean nutrients from browse and grass has not received much attention. However, experimental studies do appear to demonstrate that browsers and grazers differ in their foraging behaviour. For example, the functional responses of browsers tend to be relatively flat, whereas those of grazers appear to be asymptotic. These differences in the interaction between ruminants from the different feeding categories and their food resource are likely to lead to differences in resource exploitation and impacts on vegetation.

Full text

Browsing and grazing ruminants: are they different beasts?
Iain J. Gordon
*
Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UK
Received 1 November 2001; received in revised form 5 July 2002; accepted 28 August 2002
Abstract
Ruminant species are classified into three main feeding categories: those which feed mainly on browse material (browsers);
those which feed mainly on grass (grazers); and those which feed on a mixture of the two plant types (mixed or intermediate
feeders). Much literature has accumulated on the morphological and physiological adaptations, which have evolved in the
different ruminant feeding categories to efficiently extract the nutrients from the diet consumed. These include adaptations of the
mouth and the digestive system. Recently, there has been a number of re-analyses of the data which show that there is little
substantive evidence for differences in morphology and physiology between the feeding categories, once differences in body
mass and phylogenetic relationships have been taken into account. However, the hypotheses linking food, form and function
should not be dismissed out-of-hand until better quality experimental hypothesis testing has been conducted. In the past, the
feeding behaviour required to glean nutrients from browse and grass has not received much attention. However, experimental
studies do appear to demonstrate that browsers and grazers differ in their foraging behaviour. For example, the functional
responses of browsers tend to be relatively flat, whereas those of grazers appear to be asymptotic. These differences in the
interaction between ruminants from the different feeding categories and their food resource are likely to lead to differences in
resource exploitation and impacts on vegetation.
#2003 Elsevier Science B.V. All rights reserved.
Keywords: Ruminants; Diet; Browser; Grazer; Functional response; Digestion
1. Introduction
The consumption of food is one of the most funda-
mental activities in all animals. Evolutionary theory
hypothesizes that the ultimate goal of an organism is
to maximize its inclusive fitness, and an important
sub-goal must be to optimize food intake and diet
selection, to meet the nutrient demands of survival,
growth and reproduction. The multiple ways in which
animals attempt to meet their nutrient demands
has led to a plethora of studies, which theorize and
describe the diets consumed (Hughes, 1993). It is
assumed that animals have evolved adaptations to
ensure that they efficiently capture, consume and
digest their diets. For example, in ruminants, it is
assumed that animals, which consume browse, have
adaptations of their morphology and physiology
which differ from those of ruminant species which
specialize on a grass diet (e.g. Hofmann, 1989). These
specializations are thought to have impacts on all
aspects of the ecology and life-history of ruminant
species (Sæther and Gordon, 1994; Mysterud et al.,
2001). This review will question whether there are
indeed differences in morphological and physiological
Forest Ecology and Management 181 (2003) 13–21
*
Tel.: þ44-1224-318611; fax: þ44-1224-311556.
E-mail address: i.gordon@macaulay.ac.uk (I.J. Gordon).
0378-1127/$ – see front matter #2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0378-1127(03)00124-5

adaptations of ruminants to the different diets they
consume, and discuss ways in which animals specia-
lizing on different diets interact with their food sup-
ply. Whilst not being comprehensive, this review
does highlight the vast research that still has to be
done to understand the relationship between an
animal’s diet choice and the ecosystem in which it
lives.
The literature is replete with studies of the diets of
ruminant species (e.g. Hansen et al., 1985; Homolka,
1996; Puig et al., 1997; Mysterud, 2000) which are
generally a list of plant species consumed by an animal
species in one place at one time, seasonal changes in
diets within an animal species or differences between
animal species in diets in a given ecosystem. These
plant species lists have been used to classify ruminant
species by the diets which they consume into brow-
sers, i.e. those which feed predominantly on woody
and non-woody dicotyledonous plants (e.g. the moose,
Alces alces, and muntiac, Muntjacus reevesi), grazers,
which feed on graminaceous plants (e.g. African
buffalo, Syncerus cafer, and sheep, Ovis aries), and
intermediate or mixed feeders (e.g. red deer, Cervus
elaphus, and impala, Aeypecerus melampus), which
feed on a mixture of the two depending upon the
circumstances (e.g. location or season) (Hofmann and
Stewart, 1972); although more elaborate classification
schemes which include frugivores (Bodmer, 1990) and
obligate vs. variable grazers have been put forward
(Gagnon and Chew, 2000). This review will concen-
trate on the classification, which incorporates the
majority of ruminant species, i.e. the browser/grazer/
mixed feeder categories. This classification, along
with the more elaborate ones, are of great value in
trying to distil from the plant species lists general
principles concerning the foraging behaviour of
ruminant species that may help explain the variation
in the diets consumed. However, it is important for
scientists interested in understanding the evolution
of the dietary differences between ruminant species,
to develop both theory and empirical hypothesis
testing to elucidate the underlying mechanisms
driving species differences in the diet consumed.
To date there has been more hypothesis generation
than testing, which has led to secondary theoretical
discussions which are not based on sound statistical
grounding of empirical evidence (e.g. Hofmann,
1989).
2. Are there morphophysiological differences
between ruminant feeding categories?
Over the past three decades, a body of theory has
developed which attempts to explain why ruminant
species differ in the diets they consume, particularly in
relation to adaptations of morphology and physiology
to optimally ingest and digest different forages.
Many authors have used morphological techniques
to assess the relationship between the diet consumed
by different ruminant species and the many aspects of
the morphology of the mouth, alimentary tract, liver
and other organs (e.g. Hofmann, 1985, 1989; Axmacher
and Hofmann, 1988; Gordon and Illius, 1988; Archer
and Sanson, 2002). They then relate these morpholo-
gical adaptations to the chemical composition of plant
material and the optimal strategies for the consumption
and digestion of browse and grass diets.
The chemical composition of plants can be divided
into five major components (van Soest, 1993):
the plant cell contents which are highly digestible
by mammalian digestive enzymes;
the plant cell wall which is refractory to digestion
by mammalian digestive enzymes but can be bro-
ken down by the microflora which live in a sym-
biotic relationship with the host in the alimentary
tracts of mammalian herbivores, but this takes time;
lignin, which is a constituent of the cell wall and
which binds to the other components of the cell
wall (i.e. cellulose and hemicellulose) and limits
the digestion of the cell wall as a whole;
plant secondary compounds which act as digestive
retardants, for both dry matter and nitrogen, or
toxins;
nitrogen which occurs within the cell contents and
cell wall and which is important for the efficient
functioning of the microflora as well as providing
for the nitrogen requirement of the host.
Generally, browse plants have higher levels of cell
contents, lignin, secondary compounds and nitrogen
than do grasses (Gordon, 1989). The first consequence
of these differences in composition is that whilst cell
wall forms a greater proportion of the dry matter in
grasses, it is generally more extensively digestible than
the cell wall of browse material, although this requires
a long residence time in the fermentation chambers
housing the microflora (Illius and Gordon, 1991).
14 I.J. Gordon/ Forest Ecology and Management 181 (2003) 13–21

Secondly, whilst there are higher levels of nitrogen
in browse these can be bound to the secondary com-
pounds during chewing, making them less available
for digestion by the microflora (Robbins et al.,
1987a,b). Hofmann hypothesized that the optimal
digestion strategies for browse and grass differ, such
that browse is most efficiently digested (in terms of
maximizing the rate of return of nutrients) by limiting
the amount of time the material is retained within the
digestive tract and having adaptations to deal with
the secondary compounds (see also Hanley, 1982).
Conversely, retaining the material within the fermen-
tation chambers for a long period most efficiently
digests the poorly lignified cell wall of grass.
In support of the hypothesized differences in
optimum retention time for grazers and browsers,
Hofmann and co-workers, used the data derived from
their studies of the anatomy of a wide range of both
temperate and tropical ruminant species, to show that
the digestive tracts of grazers have structures which
appear to cause increased retention of the material in
the gut. These structures are missing or rudimentary in
browsers (Hofmann, 1973). For example, in grazers
the rumen was larger, the orifice between the reticu-
lum and omasum in the rumen larger (Demment and
Longhurst, 1987) and the salivary glands smaller than
in browsers (Hofmann, 1989). These findings closely
linked the anatomical form with the optimal function
for most efficiently digesting the types of plant mate-
rial consumed by browsers and grazers. However,
whilst the logical arguments for the hypothetical links
between form and function presented by Hofmann and
co-workers and others are compelling, recent statis-
tical analyses of the data demonstrate the limited
evidence to support the suggestions once various,
known, confounding effects are taken into account.
For example, in an extensive analysis of the data in the
literature Van Wieren (1996) and Illius and Gordon
(1999) have shown that, once the effects of body
weight are taken into account, few morphological
variables differ significantly between ruminant species
belonging to different feeding categories. However,
some results are equivocal, e.g. whilst little evidence
exists for difference between dietary categories in the
rate of passage of material through the digestive tracts
(Gordon and Illius, 1994; Robbins et al., 1995;
although see Clauss and Lechner-Doll, 2001) there
is some evidence that browsers excrete material of a
larger particle size than do grazers (Clauss et al.,
2002).
Additional support for the pervading effect of body
size on digestive strategies comes from several studies
which have shown that, after controlling for the effects
of phylogenetic relationships between species, body
weight continues to explain variation in the size of
organs, whilst, feeding category has little or no sig-
nificant effect (Perez-Barberia and Gordon, 2001;
Perez-Barberia et al., 2001). However, a meta-analysis
of the data available in the literature, controlling for
both body weight and the phylogenetic effects, sug-
gests that there may be some differences between
ruminant species from different feeding categories
in the efficiency with which they can digest the plant
cell wall of plant material of differing qualities (as
measured by the ratio of lignin to plant cell wall
(NDF)) (Perez-Barberia et al., submitted); with gra-
zers being better able to digest the cell wall across all
lignin/NDF ratios than are intermediate feeders and
browsers. This suggests that whilst there is no clear
evidence from the morphological variables measured
thus far for a difference in alimentary tract function
between browsers and grazers, the outcome of the
digestive process does appear to be different between
the species occupying different feeding categories
(Iason and van Wieren, 1999). However, the majority
of these comparative analyses draw on data for which
the confounding effects of, e.g. the type of diet con-
sumed are not controlled for and as such no firm
conclusions can be drawn about the real differences
in digestive function between browsers and grazers. It
remains for further experimentation to be conducted
where clearly defined hypotheses concerning the dif-
ferent physiological responses to diet characteristics
are tested (see also Ditchkoff, 2000). Only then will it
be possible to move the debate from one which
is based, primarily, on observation and hypothesis
generation to one, which is supported by rigorous
experimental tests of these hypotheses.
Whilst there is little evidence to support the hypoth-
esis of morphophysiological adaptation to diet con-
sumed, a hitherto under-researched area deserving
further investigation is the efficiency with which spe-
cies within the different dietary categories deal with the
secondary compounds which characterize the browse-
based diets. Firstly, intrinsic physiological adaptations
to the consumption of secondary compounds, e.g. the
I.J. Gordon/ Forest Ecology and Management 181 (2003) 13–21 15

size of the salivary glands, the proline-rich protein
content of the saliva or the activity of the mixed-
function oxidases in the liver may be greater in species
adapted to feeding on diets high in plant secondary
metabolites (Austin et al., 1989; Robbins et al., 1991).
Secondly, the microflora composition of the alimentary
tract may differ between species, which consume
different diets reflecting the physical and chemical
environment within the rumen (Hobson et al., 1975).
For example, in a broad study of the rumen microflora
of species of African ruminants, Jones et al. (2001)
hypothesized that the rumen microflora of browsing
species would be more efficient at digesting browse
material in vitro than would that of hay-fed sheep, yet,
no evidence was found to support this hypothesis in
that study. Where cases of interspecific differences in
microbial activity have been found for ruminant spe-
cies specializing on different diets (e.g. Kennedy et al.,
1987;Van Gylswyk and Giesecke, 1973;Giesecke and
Van Gylswyk, 1975;Hoppe et al., 1977a,b;Odenyo
et al., 1999) these appear to be associated with species
differences in the diet consumed, rather than animal
species-specific adaptations. As with other studies of
digestive physiology, the true test of feeding category
differences in microbial adaptation will require experi-
mental tests which control for body size, phylogeny,
intake, forage type and rearing conditions.
3. The foraging behaviour of browsing and
grazing ruminants
If there are few morphological or physiological
differences between species of ruminants, which spe-
cialize in different diets, are there other ways in which
the species differ? The following section discusses the
foraging behaviour of browsing and grazing ruminants
and will hypothesize that it is the behavioural rather
than morphophysiological component of the interac-
tion between ruminants and their food supply which
determines differences between browsing and grazing
ruminants.
3.1. The functional response
The functional response is a fundamental relation-
ship describing the interaction between the herbivore
and its food supply (Spalinger and Hobbs, 1992).
The simplest description of the functional response
is a relationship between resource abundance (e.g.
biomass, sward height) and the rate of intake of that
resource (e.g. Wilmshurst et al., 1995; Bergman et al.,
2000). Whilst the evidence is limited, it appears as
though feeding on grass and browse can produce
different forms of the functional responses. The con-
sumption of grass by grazers generally gives the
classic asymptotic relationship (Spalinger and Hobbs,
1992; Bergman et al., 2000), whilst the consumption
of browse by browsers can give a linear relationship
between the biomass and the intake rate (Renecker and
Hudson, 1986; Spalinger et al., 1988; Spalinger and
Hobbs, 1992) as well as an asymptotic one (Ginnett
and Demment, 1995). This has led to the belief that
plant biomass is a better descriptor of food availability
for grazers (Bergman et al., 2000) than it is for
browsers (Gross et al., 1993).
In a seminal paper, Spalinger and Hobbs (1992)
demonstrated that the functional response of both
grazing and browsing herbivores could be better
described by a mechanistic model of the processes
of searching, biting and chewing. They derived equa-
tions for the functional response where intake rate
was constrained by one of the three processes; i.e. the
encounter rate with cryptic food (Process I); the
encounter rate with apparent food (Process II);
and the rate at which food can be chewed and swal-
lowed (Process III). Most recent experimental tests of
the functional response in herbivore species (e.g.
Spalinger and Hobbs, 1992; Gross et al., 1993; Shipley
et al., 1999; Illius et al., 2002) suggest that both
browsers and grazers operate with Process III in which
the intake rate (I) is represented by the asymptotic
function:
I¼RmaxS
RmaxhþS
where R
max
(mg s
1
) is the maximum rate of proces-
sing plant material in the mouth that would occur in
the absence of cropping, h(s) is the average time
required to crop a bite in the absence of chewing and S
(mg DM) the bite size. The success of this model in
describing phenomena in herbivore foraging ecology
suggests that the fundamental unit of forage intake is
bite size and that understanding the foraging beha-
viour of herbivores requires us to describe the dis-
tribution of bites in the environment (see Section 4).
16 I.J. Gordon/ Forest Ecology and Management 181 (2003) 13–21

The model is held as being generally applicable
across a range of herbivore species (body size and diet
type), e.g. Gross et al. (1993) found that the model
described the feeding behaviour of a range of herbi-
vores spanning the browser–grazer continuum when
fed on alfalfa (Medicago sativa). However, there is
still a need for a true test of the R
max
and hparameters
of the model for herbivores from different feeding
categories feeding on both grass and browse material.
Given the fact that browse material can be defended by
spines (Cooper and Owen-Smith, 1986) and grass may
require greater processing by the molars than does
browse (Archer and Sanson, 2002) this experiment
would test the hypothesis that grazers are more con-
strained by R
max
for a given food type than are browsers
(because of processing constraints in the mouth) and
that the hof grazers is more constrained on browse
than is that of browsers (because of the difficulty of
selecting preferred food items).
Again, as for the studies of digestive differences
between dietary categories, in these studies it will be
important to control for the effects of body size and
phylogeny before testing for the effects of dietary
specialization. For example, when species of a range
of body weights (lemmings, Dicrostonyx groenlandi-
cus, to cattle, Bos taurus) were fed on hand constructed
alfalfa the functional response of small species appears
to reach an asymptote at a lowerplant biomass than does
that of larger species (Gross et al., 1993). This reflects
the fact that because of mouth size/body size allometry,
small species are taking a relatively larger bite at low
plant biomasses/heights than are large ones (Illius and
Gordon, 1987) and, therefore, reach their R
max
more
quickly than do large animals as plant biomass/height
increases (see also Shipley et al., 1994) (phylogeny was
not taken into account in this study).
One final point is that historically, tests of the
functional response in herbivores have been conducted
on relatively simple, often vegetative plant material
(Gordon, 1995). In order to test the effects of resource
abundance on the functional response, the impacts of
more complex vegetation, which includes both, leaf
and stem material, distributed in horizontal as well as
vertical dimensions should be assessed. This will
allow us to measure more realistic behavioural deci-
sions made by foraging ruminants and to look for
differences in the behavioural responses of ruminants
from different feeding categories.
3.2. Vegetation structure and leaf distribution
Grass and browse material presents itself in very
different ways to the herbivore. For example, leaves of
browse tend to be largeor appear in clusters (Gross et al.,
1993) and browse species tend to be more effectively
defended both mechanically and chemically than are
grasses (Cooper and Owen-Smith, 1986; Milewski
et al., 1991). On the other hand, the differences in
quality between the leaf and stem tend to be greater
in browse than in grass. As a consequence, very dif-
ferent foraging mechanics are required for browsing as
compared to grazing (Gordon and Illius, 1988), which
may mean that the components of the functional
response (i.e. R
max
and h) differ for species foraging
on grass as compared to browse. Whilst no controlled
experimental tests of this hypothesized difference in
resource presentation on feeding behaviour have been
conducted, there is evidence that animal species do
differ in their feeding efficiencies on grass and browse.
For example, in an experiment comparing the intake
rates of cattle and goats on thorny browse species which
either had their thorn or had their thorns removed, there
was a positive impact on the rate of intake by cattle but
not that of goats (Magadzire, 2002), probably reflecting
differences in mouth size and selectivity between the
species (Gordon and Illius,1988). In an extension to this
study where the intake rates of goats and cattle were
measured when foraging on grass, cattle had a substan-
tially higher intake rate on grass than they did on browse
but the intake rate of goats was similar on the two
vegetation types (Magadzire, 2002). Although the
experiments of Magadzire (2002) confound feeding
category with body size, they provides guidance for
the type of experiments required to test for differences
in feeding efficiency between ruminants, which spe-
cialize in feeding on different forage types.
3.3. Spatial distribution of resources
The scale of decision-making in herbivore foraging
has received a great deal of attention since the seminal
work of Senft et al. (1987) suggested that foraging
could be viewed as a hierarchical process ranging from
the bite to the landscape (e.g. Schaefer and Messier,
1995; Wallis DeVries et al., 1999; Apps et al., 2001;
Johnson et al., 2002). Grass and browse resources tend
to be distributed in different ways. Generally, the
I.J. Gordon/ Forest Ecology and Management 181 (2003) 13–21 17

individual browse plants are larger and more discrete
(A
˚stro
¨m et al., 1990) than are individual grass plants.
Species of grass plants also tend to be more intimately
mixed than are browse species (McNaughton, 1984).
As such, grass and browse require different degrees of
patch selection when animals are foraging for different
species or for different individuals. Consequently,
browsers may see individual trees as patches of food
resources (A
˚stro
¨metal.,1990) but grazers may see the
range of species of plants on offer in an area that can
be cropped without moving the feet (feeding station;
Goddard, 1968) as the patch. In an experiment to test
whether sheep discriminated between individual spe-
cies of grass at the feeding station scale, it was shown
that unless there were very strong preference differ-
ences between species then the sheep consumed the
grass species in proportion to their abundance (Gordon
et al., unpubl.). However, where there were large dif-
ferences in preference between the plant species then,
the sheep discriminated against the less preferred spe-
cies even when the species occurred in monospecific
patches of 3 cm 3 cm (similar to the incisor breadth
of a sheep; Gordon and Illius, 1988). This suggests that,
in general, grazers are relatively non-discriminatory
between species within a feeding station, except where
preference differences between species are high. On the
other hand, in a field-based experiment, Danell et al.
(1991) tested the scale of selection by a large browser,
the moose. Using genets of Scots pine (Pinus sylves-
tris), alder (Alnus incana) and aspen (Populus tremula)
planted in mixtures, they measured the selection by
free-ranging moose at the stand and tree scale. Their
results were most effectively explained by the moose
selecting at the level of the tree, rather than at the level
of the stand. Whilst more rigoroustests of the difference
in scale of selection between browsers and grazers are
required, it can be hypothesized that browsers are more
selective for different individual plants within species
of browse (Duncan et al., 1994; Hartley et al., 1997)and
within individual plants than are grazers, which will
tend to be selective for the mixture of grass plants
available at the feeding station level.
4. Resources as perceived by grazers
and browsers
The previous section of this review outlined differ-
ences in the way in which browsers and grazers interact
with their resource. This reflects the distribution of
browse and grass material in both the horizontal and
vertical dimension, and the morphology and chemistry
of the plant material itself. To date there has tended to
be a rather simplistic approach to understanding the
interaction between mammalian herbivores and the
vegetation resources upon which they feed. For exam-
ple, much of the ecological research on foraging has
looked at instantaneous intake rate and diet selection
(Hughes, 1993; Hodgson and Illius, 1996); whilst
animal scientists have looked at diets in terms of their
nutritional value (Forbes, 1995). Illius and Gordon
(1990) advocated a more holistic approach to under-
standing feeding behaviour, which linked the periph-
eral sensation (sight, smell), ingestion, digestion and
absorption components of the resource acquisition axis
via the perception of the animal. Whilst some attempts
have been made to take this more holistic approach,
there is still a need for herbivore foraging ecologists to
develop a much greater understanding of the relation-
ship between an animal’s morphology and physiology
and its perception of the resources in the environment
that it occupies.
For any given ecosystem, browsers and grazers are
likely to have a very different perspective on the
distribution of resources and as such are likely to
distribute themselves and move around differently.
The consequence of this will be that browsers and
grazers have very different impacts on ecosystem
processes. Whilst little research has been done on
the perception of resource distribution by different
animal species, several studies do point to the impor-
tance of plant- and animal-based factors on the ways in
which animals interact with their food resources. Fritz
et al. (1996) found that the group size of impala
(Melampus aeypercerus) determined the sizes of Aca-
cia bushes that groups selected to feed on, suggesting
that the perception of the resource by individuals in an
impala group was strongly affected by social factors.
In another example, Etzenhouser et al. (1998) found
that despite the fact that they are both browsers, the
movement patterns of goats (Capra hircus) and white-
tailed deer (Odocoileus virginianus) responded differ-
ently to the distribution of browse plants in a Texas
shallow ridge range site, with goats being more influ-
enced by overall plant community structure whereas
white-tailed deer were more affected by the distribu-
tion of their preferred forage species. Finally, in an
18 I.J. Gordon/ Forest Ecology and Management 181 (2003) 13–21

excellent, overlooked, comparative study of the feed-
ing behaviour of a community of African ungulates in
a deciduous miombo savanna woodland, Underwood
(1983) demonstrated that mixed feeders tended to
move faster and spend less time feeding at a feeding
station than did the grazers. He also found that the
dichotomy between feeding categories in the response
of movement rates and feeding station residence time
were greater for small species than they were for large
species. Unfortunately, there were not enough species
of browsers in this study to conclude anything from
their response to the distribution of resources in the
environment. However, the study does demonstrate
that less of the plant material in the environment
studied is acceptable as food for the intermediate
feeders than is the case for the grazers.
5. Conclusions
Much insight has been gained from the classifica-
tion of mammalian herbivores into those, which con-
sume a predominately browse-based diet as compared
to those, which consume a predominately grass-based
diet. Whilst attempts to link this classification with
functional aspects of the morphology and physiology
of ruminants have recently been challenged by rigorous
analyses of the data, future hypothesis testing using
controlled experiments will be invaluable in moving
our understanding of the determinants of diet composi-
tion in ruminants forward. For example, experimenta-
tion is required to test the differences in cell wall
and dry matter digestibility for a range of different
forages between animals, which consume different
diet types controlling for body size and phylogenetic
effects. Similarly, a key experiment would test R
max
and hacross a range of different forages for ruminant
species, which feed on different diet types.
It is clear that grass and browse do differ in their
chemistry, morphology and distribution in the land-
scape, and it is likely that diet choice and foraging
behaviour of ruminant herbivores will differ in rela-
tion to the resources upon which are fed. In future,
more emphasis needs to be placed on understanding
the ways in which animals, which consume different
diets, interact with, perceive and respond to the
resources in their environment. For example, for a
given ecosystem, browsers and grazers and large and
small species are likely to take a different view as to
which plants offer adequate nutrient intake rates and
this will affect their perception of the distribution
of resources in the landscape. As a consequence,
different species are likely to respond differently to
management and environmental change.
Finally, the ultimate outcome of much of the
research conducted by herbivore ecologists is the
ability to predict the consequences of herbivory for
the vegetation (Armstrong et al., 1997a,b;Reimoser
et al., 1999). Clearly, it is important for those inter-
ested in the animal component of plant–herbivore
interactions to link more closely with the plant ecolo-
gists studying the impacts of herbivory on the plant
growth, survival and reproduction.
Acknowledgements
Thanks go to Glenn Iason, Javier Perez-Barberia,
Lucy Burnett and two anonymous referees for com-
ments on an earlier version of this manuscript. The
Scottish Executive Environment and Rural Affairs
Department supported this work.
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